1. Field of the Invention
The present invention relates generally to the field of corrected analog sensing systems and specifically to an analog transducer sensing system which includes temperature gain and offset as well as transducer gain and offset corrections.
2. Background of the Prior Art
Measurements obtained from transducers or sensors are affected by inherent static errors as well as thermal drifts. The static errors are in the form of linearity, repeatability and hysteresis effects which are a function of the specific transducer and transducer type. Thermal errors result in zero offset and sensitivity changes over the operating temperature range of the transducer which, in the case of piezoresistors, can be highly nonlinear.
Sensors employing various measurement parameter transducers historically have been used to provide an electrical output signal which is related to changes in the parameter being measured. Many times this parameter is the strain on a particular structure and, depending upon the mechanical arrangements of the structure, can be indicative of acceleration, torque, pressure, etc. More recently, there has been significant growth in microtransducers which utilize micromachining of silicon to provide extremely small pressure and acceleration sensors. These sensors employ piezoresistive areas on the silicon to provide a variable resistance output as the silicon is strained.
The outputs of such transducers are generally non-linear and offset. By linearity (or gain) of the transducer output is meant the relative change in output reading for a given change in the measurement parameter and such linearity can change as a function of the transducer output. By offset, it is meant the transducer output with a zero or reference measurement parameter input which is a function of temperature. This can be expressed by the equation: electrical output=a(mechanical input)+offset where "a" may be a function of the mechanical input itself ("a" would be a constant if the transducer were linear). Furthermore, there can be temperature related gain effects ("a" changes as a function of temperature) as well as temperature related offset effects which will change the output reading of the transducer without any variation in the measured parameter. Unfortunately, many transducers have nonlinear outputs and also have variations in signal offset depending upon the level of the parameter to be measured.
It is highly desirable that such transducers provide a linear output signal which is proportional to the parameter being measured over the useful range. Because of the above described temperature gain and offset errors, as well as the measurement parameter gain and offset errors (which are a function of the transducer), require extensive calibration of measurement transducers over the entire operating range of temperatures and parameters being measured. These are generally in the form of correction tables which are then utilized to manually or with computer implementation, correct the sensor output.
While the output from such sensors is in an analog form, traditional analog compensation of these errors is extremely time consuming and is not very accurate especially for nonlinear errors. Attempts have been made in the past to improve upon such systems and are exemplified by U.S. Pat. No. 4,399,515 issued to Gross on Aug. 16, 1983 and U.S. Pat. No. 4,912,397 issued to Gale et al. on Mar. 27, 1990.
In the Gross patent, there is shown in FIG. 1 a bridge type silicon diaphragm pressure sensor in which temperature induced linearity (or gain) errors and temperature offset errors are corrected as a result of the output of offset DAC (Digital-to-Analog Converter) 16 being used to correct sensor amplifier 14. Thermal induced changes in the sensor itself are taken into account by the thermal shift DAC 21 and its output is applied to programmable gain output amplifier 22. It is noted that the thermal shift and the sensitivity shift analog outputs, which are supplied as inputs to programmable gain output amplifier 22, are all based upon the digital temperature address provided by ADC (Analog-to-Digital Converter) 20.
The drawback of such a system is that, while it preserves the benefits of an analog signal bandwidth, it has no ability to correct transducer gain and offset errors which can be more significant than temperature related gain and offset errors.
The Gale patent teaches, in FIG. 2, the linearization or correction of a non-linear transducers output by conversion of one portion of the output to a digital signal which addresses memory 4. The digital data stored in the memory is accessed by the digital address and then reconverted into an analog correction. The correction is applied to summer 6 which includes as another input the transducer output and provides a corrected linear output signal. There is no indication that this correction includes "offset" for the transducer and there certainly is no provision for temperature induced linearity correction or temperature offset correction.
Neither the Gale patent nor the Gross patent provide a system for complete correction of a transducer analog output signal. It is particularly important to maintain the signal as an analog signal so as to preserve the relatively broad bandwidth of such a signal. If the signal is converted into a digital signal or address which is then applied to a digital correction table so as to read out a corrected digital signal output, even if this readout from a digital memory system is reconverted into an analog output signal, the bandwidth of the output signal is immediately decreased to the range of the digital memory and speed at which the digital memory can supply outputs reflecting changes in the inputs. Such a conversion to a digital signal and then reconversion back to the analog signal after correction provides a relatively poor accommodation to variations in temperature and transducer manufacturing effects.
Furthermore, when temperature effects on a transducer are to be corrected for, as in Gross above, often the temperature sensor is located a physical distance from the transducer and thus may not accurately indicate the actual temperature of the transducer.